1. Technical Field
The present invention relates to silicon carbide-based ceramic systems and more particularly to improving the toughness of silicon carbide-based ceramic systems.
2. Discussion
Ceramics are growing in popularity as a structural material for many applications, such as heat engines. One drawback encountered in the ceramic field is a difficulty in improving the toughness of the ceramic material. A popular ceramic system is based upon silicon carbide (SIC). SiC-based systems are popularly employed in many applications. A conventional .beta.-SiC material exhibits a typical fracture toughness (K.sub.IC) such as about 2.7.+-.0.2 Mpa-.sqroot.m. Unfortunately, this often may not be sufficient for a number of commercial applications, such as certain heat engine applications. It is, therefore, desirable to improve the toughness of SiC-based systems.
The existence of a series of aluminum nitride (AlN) rich solid solutions in the system Si.sub.3 N.sub.4 --SiO.sub.2 --AlN--Al.sub.2 O.sub.3 has been reported L. J. Gauckler, H. L. Lukas and G. Petzow, Contribution to the Phase Diagram Si.sub.3 N.sub.4 --AlN--Al.sub.2 O.sub.3 --SiO.sub.2 O.sub.3, J. Am. Ceram. Soc., 58 [7] 346-347 (1975); see also, D. P. Thompson, P. Korgul and A. Hendry, "The Structural Characterization of SiAlON Polytypoids" in "Progress in Nitrogen Ceramics" (Edited by F. L. Riley) NATO ASI Series, Series E: Applied Sciences--No. 65, pp. 61-74 (1983). Some of the polytypoids have been recognized as having an elongated plate like morphology. An AlN-ceramic with a fibrous microstructure of 27R polytypoid has been fabricated using SiO.sub.2 Y.sub.2 O.sub.3 additives. See, K. Komeya, H. Inoue and A. Tsuge, "Role of Al.sub.2 O.sub.3 and SiO.sub.2 Additions in Sintering of AlN," J. Am. Ceram. Soc., 57 [9] 411-412 (1974); and K. A. Schwetz, H. Knoch and A. Lipp, "Sintering of Aluminum Nitride with Low Oxide Addition" in "Progress in Nitrogen Ceramics" (Edited by F. L. Riley), NATO ASI Series. Series E: Applied Sciences--No. 65, pp. 245-252, 1983.
The solid solution formation of SiC-AlN has been reported. Rafaniello, W., "Fabrication and Characterization of Silicon Carbide Alloys: The Silicon Carbide-Aluminum Nitride System," Doctoral Dissertation, The University of Utah, (1984). Other related literature includes J. W. McCauley and N. D. Corbin, "High Temperature Reactions and Microstructures in the Al.sub.2 O.sub.3 -AlN System," NATO ASI Series. Series E: Applied Sciences--No. 65, pp. 111-118, 1983; S. Shinozaki and K. R. Kinsman, "Evolution of Microstructure in Polycrystalline Silicon Carbide," Proceedings of Crystalline Ceramics; Edited by Hayne Palmer III, R. F. Davis, and T. M. Hare, pp. 641 (1978); and R. Ruh and A. Zangvil, "Composition and Properties of Hot-Pressed SiC-AlN Solid Solutions," J. Am. Ceram. Soc., 65 [5] 260-265 (1982).